does that mean that i will have to take the ecu from the allroad or would the original one need to be remapped in the a4
I'm no expert on this and I may be wrong, but I've played around with engines & ECUs from the Peugeot Citroen range some years back:
It didn't take me long to realise that the 1.9i petrol engine in the ZX Volcane (hot-hatch) was expensive, and its ECU (mounted towards the front and easily damaged in a shunt) was even more expensive (I was quoted £200 secondhand, and even then only when one came into the yard). However the Peugeot 406 SRi (a popular rep-mobile) also had a similar 1.9i petrol engine and its ECU was a mere £10 secondhand with about 50 to choose from.
Following a shunt in my Citroen rally car which the engine survived but the ECU didn't, I quickly bought one from a Peugeot breaker and stuck it in. The result was a slightly different power and torque curve, which I could feel whilst driving, but everything ran fine and I never did bother to change it again!
Based on this experience therefore I'd say that if both engines are VAG AKE there's a good chance they use similar sensors and ancilliaries, so you can probably pick whichever ECU you would prefer the characteristics of and drive using that. In this sense you are perhaps doing very little different from the company that charges you money to burn a different program onto your existing ECU, except that you're choosing a different factory program (and probably saving money too).
This is just my opinion however, based on experience with a different maker's petrol engines, so it's entirely your risk and don't blame me if it all goes horribly wrong and costs lots to fix!
